When you look at a gas either draw this little grid or visualise it with your mind’s eye:

For instance, if PaCO2 is 50 mmHg, that’s an increase of 10 mmHg from normal (PaCO2 40 mmHg). Thus in acute metabolic compensation the HCO3 should increase by 1 mmol/L from normal (HCO3 25 mmol/L) to 26 mmol/L, and in chronic compensation it should increase by 4 mmol/L from normal to 29 mmol/L.

Using the 4 square grid helps you remember which numbers correspond to what (acute comes before chronic, left before right), and whether they go up (top row) or down (bottom row).